Abstract

“Optimisation of the choice of parameters for an operational off-line retrieval of vertical profiles of trace gases from MIPAS data”.
Since March 2002 the environment satellite ENVISAT is operational. ENVISAT carries a series of instruments used for remote sensing the earth atmosphere. MIPAS, as one of those instruments is, due to its conception as a limb sounding infrared spectrometer, intended to provide new insights in the chemical processes of the middle atmosphere. To achieve this MIPAS measures emissions of trace gases like H2O, O3, CH4, NO2, N2O, HNO3. Additionally MIPAS acquires global pressure and temperature profiles.
Vertical profiles of trace gases are computed from calibrated measurements in the processing chain by an iterative calculating scheme (retrieval). To this measurements are compared with simulated spectra from a forward model. At each comparison modelling parameters are systematically adjusted until an agreement in the compared quantities is reached, which lies within the boundaries of some previously defined criteria. Improvements in the retrieval scheme are possible in three categories: first, adjustments in the side constraints (regularisation), which divide the mathematical possible solutions from the physical reasonable, are conceivable. Second, adjustments in the handling of certain systematic errors are necessary and finally, one is interested in eliminating measurements which have become useless due to cloud influences.
To obtain a measure for the improvements, some criteria are defined and discussed in this work, with which it is possible to compare differently parameterised retrievals with regard to their quality. These criteria are: height resolution (averaging kernel), information content and degrees of freedom, as well as common quantities like root mean square.
The subject regularisation is being discussed by the means of a comparison between a newly developed approach (HoMa), a not yet with operational trace gas retrievals used maximum entropy method (MEM2) and the standard methods Optimal Estimation and Tikhonov.
It can be shown, that the alternative methods have massive advantages compared with the standard methods in terms of the above defined criteria. This also is true for aspects like transparency, diagnostic options and quality of the retrieved profiles. Especially the results of MEM2 have to be pointed out, because it does not use a priori knowledge.
The treatment of systematic errors shows, that this is an essential part of every retrieval. The results of computations with simulated data while using the corrections show tremendous improvements compared to results of not corrected computations.
The part which deals with the consideration of cloud influences in measurements shows the advancement of an established idea. A detailed analysis of cloud indices enables to rescue measurements which otherwise would have been parcelled out for security reasons. This may lead to gather additionally knowledge about tropospheric regions from MIPAS measurements.
A comparison of results of the work with independent results from the ROSE-model delivers promising consistence.